The physiological and subjective changes produced by inhaled doses of nicotine will be studied in frequent and infrequent tobacco smokers. Tolerance development and abstinence phenomena will be evaluated. In two successive, day-long laboratory sessions, smokers will smoke tobacco cigarettes containing between zero and four miligrams of nicotine every half hour for eight hours. Their blood nicotine and cotinine levels will be monitored and their brain wave activity, cardiovascular functioning and subjective reactions will be recorded. Tolerance development will be followed and inferred from diminished response magnitudes and from the reproduction of initial effects following the smoking of a higher nicotine dose. The time course and inensity of nicotine abstinence phenomena will be studied while subjects smoke placebo cigarettes. The final cigarette of the session will contain nicotine and its effect on reversing the abstinence symptoms will be evaluated. Detailed smoking and drug use histories, health histories and self-reports of reasons for smoking and personality traits will be obtained prior to the subjects' participation and will be used in conjunction with the laboratory data to develop psychophysiological profiles of the smokers. The profiles will be used to devise selection criteria whereby appropriate treatments can be matched with the particular individual qualities of a smoker wishing to stop smoking.